1. Field of the Invention
The present invention relates to a defect correcting apparatus and a correct method for a photo mask (to be denoted as a reticle hereinbelow) that is used in a lithographic process with a reduction-projection exposure apparatus for use in manufacturing semiconductor devices.
2. Description of the Related Art
Recently, in the lithographic process used for manufacturing semiconductor devices, reduction-projection exposure equipment is generally used in order to deal with the formation of micro patterns. In a reduction projection exposure apparatus, a reticle having a pattern that is, for example, four times as large as the photoresist (to be denoted as “resist” hereinbelow) pattern to be formed on a semiconductor substrate surface is used as an exposure mask. Usually, one reticle is used for manufacturing more than tens of millions of semiconductor devices. The quality of the patterns formed on the reticle, therefore exerts a great influence on the production yield of the semiconductor devices. The quality of the pattern is essentially determined depending on the presence or absence of pattern defects. This is why the defect inspection on reticle patterns is a requisite stage to improve the production yield of semiconductor devices.
Further, various pattern correcting technologies such as phase shift techniques, optical proximity correction and the like for forming extra fine patterns are demanded for the production of reticles that are used for recent highly integrated semiconductor devices, and realization of such technologies needs an enormous amount of information processing. As a result, the production cost for a reticle has become very expensive. In order to prevent an increase in cost, a good deal of effort is directed towards using all possible correcting means to ensure that reticles that pattern defects can be made usable.
FIG. 1 shows a flow chart for explaining conventional steps for correcting defects of a reticle. A reticle that is finished with pattern formation is set on a visual inspection device so as to check whether there is any defect (Step S1). When a pattern defect is found, the pattern defect is located (Step S2). Then, it is determined whether the pattern defect can be corrected (Step S3). If there exists a large defect that affects a large number of patterns, the defect is determined to be uncorrectable (Step S4), and the reticle is determined to be a failure and discarded. On the other hand, when the defect is determined to be correctable, then the reticle is set on a pattern defect correcting apparatus, and the correcting conditions are determined (Step S5). Subsequently, correction of the pattern defect is carried out based on the correcting conditions (Step S6). Then, the corrected reticle is set on an optical microscopic type wafer exposure optical emulation system that includes an illumination optical lens system equivalent to that of the wafer exposure device and a projection optical lens system capable of providing a resolution equivalent to that of the wafer exposure device, and the image of the corrected defective pattern portion is observed (Step S7). The optical information obtained from the image-forming surface in the aforementioned emulation system is displayed as an optical image of the defect on the reticle with a resist pattern (transferred image), which is transferred onto the wafer by the actual wafer exposure device.
Next, from the wafer transferred image of the defective pattern portion, obtained at Step S7, the maximum light intensity, minimum light intensity, contrast and steepness of the optical steep characteristic of the above optical image are obtained, and the variational values of the image size of the defective portion calculated based on these pieces of data and the threshold of light intensity that is uniquely determined from the target resist size are determined. Then these size variational values are compared to the permissible specifications of the product so as to determine whether correction is permissible (Step S8). When it was determined that the correction level has not reached the permissible specifications due to an insufficient amount of correction, the operation returns to Step S5 again, and additional correcting conditions are set up based on the operator's empirical rule, taking into consideration the result at Step 7 in comparison with the correcting conditions set at previous Step S5.
Thereafter, Steps S6 and S7 are executed, and the result of correction is evaluated once again. This correction routine of steps is repeated until the result falls within the desired level of correction. Only when the result meets the permissible specifications of the product at Step S8 after the latest Step S7, the reticle is determined to be acceptable, and the correction of the reticle pattern defect is completed (Step S9). On the other hand, when overcorrection was made so that the result exceeds the permissible limit level of the production permissible specifications, and it is determined at Step S8 that there is no way to make a restoration from the excessive correction to the initial condition, this situation is regarded as an event in which continuation of the reticle fabrication is impossible, and the operation goes to Step S4, where the result is determined to be a failure.
The reticle pattern defect inspection and defect correcting method as above are disclosed in Japanese Patent Application Disclosure No. 516898/2001 and Japanese Patent Application Laid-open No. 037579/2004.
Reticle pattern defects, as the target of the above correction, are roughly classified into two categories. One is a pattern defect that is generally called a black defect, such as a projective defect that juts out from an inherent device pattern edge, an isolated defect remaining at an area where there should be no pattern, and the like. The other one is a pattern defect that is generally called a white defect, such as a crack defect in a pattern edge area (mouse-bite), a micro opening defect of a pinhole and the like.
Correction of the former one, i.e., a black defect, is carried out by a sputter etching process in a vacuum using a FIB (Focus Ion Beam) apparatus or by a micro machining technique by which the defective portion is physically crushed under normal pressure by an extra fine needle (cantilever) and the broken pieces are removed. On the other hand, correction of the latter one, i.e., a white defect, is generally carried out by an ion beam deposition method whereby a shading film is deposited on the defective portion by a FIB apparatus.
When a high degree of difficulty is expected to correct a pattern defect on a reticle, the amount of correction at one time is set to be small so as to improve the accuracy of correction at Step S6 in FIG. 1. That is, taking into account the result of correction at Step S7 that correspond to the correcting conditions set up at the previous Step S5, step-by-step correction and the inspection routine of the corrected result is repeated.
Referring now to FIGS. 2(a) to 2(f), the above step-by-step correction case will be described taking an example in which there exists a projection defect between two line patterns on a reticle. FIG. 2(a) shows a state in which projection defect 2 exists between two line patterns 1. Here, line pattern 1 is formed of a shading chromium layer having a light transmittance of 0% while the white part in the drawing is assumed to be of glass. To begin with, the reticle is set on an inspection apparatus such as an optical viewer having a high resolution, SEM (Scanning Electron Microscopy) or the like, so as to measure the position of projection defect 2 and defect size 7. Then, an optimal defect correcting apparatus is selected taking into account the condition of the defect. When a black defect exists close to dense device patterns and when a high accuracy correction is demanded, as in this example, a micromachining technique using a cantilever is employed. After the reticle is set on a micromachining apparatus, the first correcting conditions are set and correction of projection defect 2 is effected. Then, the reticle is set on the inspection apparatus once again, and defect size 8 of the first correction result shown in FIG. 2(b) is estimated so as to determine whether another correction is needed. In this example, since defect size 8 is not equal to size 11 (FIG. 2(e)) of the normal part, it is determined that the last correction is insufficient, and an additional correction is carried out. The reticle is set on the micromachining apparatus once again, and a correction is made based on the correcting conditions that have been newly set up. Since, in FIGS. 2(c) and 2(d), corrected defect sizes 9 and 10 are not equal to size 11 of the normal part, it is determined that a further additional correction is needed. This correcting routine is repeated until the final corrected result converges to defect size 11 in FIG. 2(e).
Here, as shown in FIG. 2(f), if corrected part 6 has defect size 12, the projection defect is excessively corrected. Accordingly, it is determined that the current correcting routine cannot be continued any longer.
As above, in the related defect correcting method, estimation of the defect size, setup of the correcting conditions, observation of the corrected result, determination of whether the corrected result is acceptable or not, are performed separately. As a result, in the series of correcting steps, from Steps S5 to S8 shown in FIG. 1, it is impossible to achieve efficient correction since there is no other way to rely on the recurrent convergence method based on an empirical rule. Since the process depends on an empirical rule, the estimate of conditions for additional correction in the next loop is mere expectation based on the previous correction result. For this reason, the final correction results in overcorrection due to expectation errors, causing the problem of convergence impossible. Further, since it is hence difficult to grasp the processing capacity because the required time for completing the correction and the number of total steps are not constant, this method faces difficulties in taking measures to reduce the time for correction.
Next, as another problem with a related defect correcting method, an example of correcting a defect in a reticle pattern made up of a halftone phase shift mask will be described with reference to FIGS. 3 to 7.
FIG. 3(a) shows a reticle pattern made of a halftone phase shift mask. Pattern 13 forming a shading portion has an incident light transmittance of 6 to 8%, and is made of a translucent film that reverses the phase of its diffracted light or shifts the phase by 180 degrees relative to the glass portion having an incident light transmittance of 100%, around the pattern. There exists white defect 14 in a part of pattern 13. FIG. 3(b) shows transferred pattern 13a when the reticle pattern of FIG. 3(a) is transferred to the wafer. Since the light intensity at the part of white defect 14 is high, wire break 1 arises around the area corresponding to white defect 14 in the transferred image.
In contrast, FIG. 3(c) shows a case where black defect 16 exists in a part of pattern 13 on the reticle. FIG. 3(d) shows transferred image 13a of FIG. 3(c). Bridging 17 arises between the patterns around the area corresponding to black defect 16 in the transferred image.
As described above, when white defect 14 shown in FIG. 3(a) and black defect 16 shown in FIG. 3(c) are corrected, the black defect can be corrected by removing the projected part. On the other hand, to correct white defect, chromium film is film locally formed around the cutout first, then the unnecessary projected part of the chromium film is removed. Since chromium film has a light transmittance of 0%, a defect corrected part consisting of chromium film having a light transmittance of 0% will remain in the translucent film having a light transmittance of 6 to 8% after correction of the white defect. This coexistence of films having different light transmittances makes it difficult to correct a halftone phase shift mask, especially a white defect. Next, the reason correction of a white defect in a halftone phase shift mask is made difficult will be described.
FIG. 4 shows a process in which chromium film is film locally formed over white defect 14 shown in FIG. 3(a) and then corrections are made step by step. FIG. 4(a) shows a state in corrected portion 18 directly after chromium film is film locally formed. FIG. 4(b) shows corrected portion 19 that has been corrected up to a condition in which a marginal projection remains along the pattern edge of the normal portion. FIG. 4(c) shows corrected portion 20 that has been corrected up to the position of the pattern edge of the normal portion. FIG. 4(d) shows corrected portion 21 that has been excessively corrected to a condition in which an indentation into the patter is formed from the pattern edge of the normal portion.
FIG. 5 shows the simulated results of the transferred images corresponding to the cases in FIG. 4. Simulation was implemented using a line-and-space pattern at a pitch of 400 nm on a reticle including a defect or a defect corrected portion, under exposure conditions with a wavelength of 193 nm, a numerical aperture (NA) of 0.85, σ out of 0.85, 2/3 annular illuminator and a reduction ratio of 1/4. Here, σ out corresponds to the outer periphery of the aperture on the pupil surface of the projection lens, and σ in corresponding to the inner periphery of the aperture resides at a position two thirds of the σ out from the center of the lens. That is, σ in is 0.57. Further, if required, it is possible to perform simulation with a higher accuracy by including the information on photoresist such as absorbance, refractive index, acid diffusion length and the like.
FIG. 5(a) shows transferred image 18a corresponding to corrected portion 18 in FIG. 4(a). FIG. 5(b) shows transferred image 19a corresponding to corrected portion 19 in FIG. 4(b). FIG. 5(c) shows transferred image 20a corresponding to corrected portion 20 in FIG. 4(c). FIG. 5(d) shows transferred image 21a corresponding to corrected portion 21 in FIG. 4(d). The noticeable point with these results is that if corrected portion 20 in FIG. 4(c), that was corrected in agreement with the geometrical position of the regular portion on the reticle, is transferred, the result pattern on the transferred image will not form a normal pattern but presents a narrower pattern than the normal pattern. What is transferred correctly on the transferred image is a reticle pattern having corrected portion 19 shown in FIG. 4(b), which has to be further corrected on the reticle. This phenomenon is caused by the fact that a direct corrected portion consisting of chromium film having a transmittance of 0% remains in a translucent film having a transmittance of 6 to 8% after the correction of a white defect on a reticle formed of a halftone phase shift mask.
FIG. 7 shows a relationship between the positional deviation at the corrected portion on the reticle and that on the transferred image, summarized from the above results shown in FIGS. 4 and 5. The lateral axis represents the amount of deviation from the normal position of a pattern on the reticle and the vertical axis represents the amount of deviation from the normal position in the transferred image. Each axis includes positive and negative deviations. A negative deviation on the lateral axis corresponds to deviation 23 that occurs when the edge of defect corrected portion 21 of reticle pattern 13 shown in FIG. 6(a) resides inside normal portion edge 22. A positive deviation on the lateral axis corresponds to deviation 24 that occurs when the edge of defect corrected portion 18 of reticle pattern 13 shown in FIG. 6(c) resides outside normal portion edge 22. On the other hand, a negative deviation on the vertical axis corresponds to deviation 23a from normal portion edge 22a of transferred pattern 13a shown in FIG. 6(b) and a positive deviation on the vertical axis corresponds to deviation 24a from normal portion edge 22a of transferred pattern 13a shown in FIG. 6(d).
The data point designated at 18b in FIG. 7 corresponds to corrected portion 18 shown in FIG. 4(a), and for a deviation of 80 nm on the reticle, the deviation of corrected portion 18a on the transferred image shown in FIG. 5(a) is 25 nm. Similarly, the data point designated at 19b corresponds to corrected portion 19 shown in FIG. 4(b), and even though a deviation of 35 nm remains on the reticle, the deviation of corrected portion 19a on the transferred image shown in FIG. 5(b) is 0 nm. Also, the data point designated at 20b corresponds to corrected portion 20 shown in FIG. 4(c), and even though the deviation is 0 nm or even though no deviation remains on the reticle, the deviation of corrected portion 20a on the transferred image shown in FIG. 5(c) shows −15 nm. Further, the data point designated at 21b corresponds to corrected portion 21 shown in FIG. 4(d), and for a deviation of −20 nm on the reticle, the deviation of corrected portion 21a on the transferred image shown in FIG. 5(d) is −23 nm.
The above result demonstrates that there is a variance between the corrected state on the reticle and the state on the transferred image.
As described above, when using halftone phase shift masks, which will become essential in the future development of pattern formation into more extra fine configurations, even if an exact physical shape correction is done on a reticle, the transferred image of the reticle pattern, which is projected on a wafer by light rays, cannot be necessarily corrected exactly. Accordingly, it is necessary to reduce the influence of the defect by checking the corrected state of the transferred image with an inspection device every time the defect on the reticle is corrected by a defect correcting apparatus. For this reason, defect correction on a reticle requires a long time, hence this is one cause for the significant reduction in production yield. Further, in an extreme case, an excessive correction makes it impossible to restore the reticle itself, resulting in no alternative but to discard the reticle, causing the problem of a large cost loss.